Drive unit, preferably an actuator, a control and a construction

ABSTRACT

In particular in articles of furniture having at least an element which may be adjusted with a DC motor, preferably via a mechanical transmission, it is desirable to have a level of acoustic noise as low as possible. The power supply to the DC motor is based on a transformer with rectification and smoothing. With the invention it has surprisingly been found that the level of acoustic noise may be reduced significantly in that the secondary side of the transformer has a first control to compensate for the loss in the motor, thereby keeping the speed thereof constant for a long period of time, and a second control adapted to remove the ripple in the voltage, thereby keeping the speed of the motor constant for a short period of time.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a drive unit for moving parts ofarticles of furniture.

2. The Prior Art

Actuators driven by a reversible low volt DC motor, typically 24V, areused for adjustable articles of furniture, such as beds, chair andtables. In a linear actuator, the motor, via a transmission, drives aspindle having a nut fixed against rotation so that the nut can move toand fro on the spindle depending on the direction of rotation of thespindle. The nut has arranged thereon a tubular rod whose free end isprovided with an attachment for securing in the structure. Typically,the transmission simply consists of a worm gear, where the worm isprovided in an extension of the motor shaft and the worm wheel secureddirectly on the spindle. An example is disclosed in WO 02/29284 to LinakA/S. A special structure of linear actuators is called lifting columns,e.g. intended for table legs. Rotary actuators have so far not been ascommon as linear actuators in connection with furniture, but a singleexample based on a specially constructed planetary gear is described inWO 01/17401 to Linak A/S. Also known are other forms of drive unitsconstructed specifically for use in furniture. In the furniturebusiness, the price of the actuators is a decisive factor, which hasbecome particularly pronounced recently. This, of course, has resultedin a development toward simpler structures. Thus, it may be mentionedthat for cost-reducing purposes the power supply is dimensioned in viewof the circumstance that it is loaded only briefly when the article offurniture is adjusted. Or put differently, it is allowed to overload thepower supply briefly. For example, in case of a height-adjustable tablewhich may be adjusted from 650 mm to 1250 in the height, it takes 12seconds to get from one extreme position to the other at a speed of 50mm/sec. To avoid continuous overload operation, running is just allowedfor a given interval of time so that the power supply is notsuperheated.

Especially in the furniture business, a decisive aspect is the level ofnoise, and with a demand for less expensive actuators it is even moredifficult to meet the requirement for low-level noise actuators.

During adjustment of the article of furniture mechanical noise occurs,such as transmission noise, noise in the suspension of the adjustableelement, noise because of vibrations that propagate in the structure,etc. When the most powerful single sources of noise have been remedied,it is a complex noise picture that emerges, it being very difficult, ifnot impossible, to locate the individual sources of noise.

In case of furniture, the level of noise is a significant factor in thesense that this must be as low as possible. This applies to beds,including hospital beds and sickbeds, as well as armchairs and tables,including desks. In the furniture structure and in the drive unit,various common noise-reducing measures may be taken, such as the use ofrubber/plastics suspensions, plastics bushings, lubricants, etc. tocounteract the noise, but in spite of this it is still desirable toreduce the level of noise additionally.

Some of the noise originates from the motor, where part of the noise iscaused by the structure of the rotor with axis-parallel air gaps betweenthe iron flanges on which the coils are wound. This gives small, butnoticeable discontinuities in the magnetic field that cause vibrationswhich propagate in the structure and cause acoustic noise. To obviatethe discontinuity, it has been proposed to twist the armature so thatthe air gap is not axis-parallel. The phenomenon is described in anotherconnection in U.S. Pat. No. 4,616,151.

Another part of the noise from the motor is caused by an axial movementof the rotor, which likewise results in vibrations and thereby acousticnoise. Owing to manufacturing tolerances it is difficult to do anythingabout the problem. The phenomenon is also described in anotherconnection inter alia in U.S. Pat. No. 5,497,039 and in U.S. Pat. No.6,069,422, both of which propose solutions for fixing the rotor in theaxial direction via the magnetic fields of the stator.

Another aspect of the acoustic noise is the transmission noise, part ofwhich originates from the engagement between the gear wheels or moreparticularly the varying degree of the engagement. In this context,reference may be made to WO 01/94732 A1 to VKR Holding A/S, whichaddresses noise problems in relation to window openers, focus being onnoise caused by clearance between the components of the transmission, itbeing attempted to reduce the noise by controlled engagement between aworm and a worm wheel.

The outlined solutions are expensive and complicated or cannot readilybe implemented in the present connection.

The invention is based on an ever increasing wish for reducing the noiselevel of linear actuators, bearing in mind that this must not lead toincreased or appreciably increased costs of these.

SUMMARY OF THE INVENTION

By the invention it has surprisingly been found by chance that thegeneral level of acoustic noise may be reduced significantly by purelyelectrical means. This is brought about according to the invention byconstructing a drive unit which includes a DC motor having a rotor witha plurality of coils connected to a commutator in connection with a setof brushes to establish a voltage across the coils, the DC motor, via atransmission, driving an adjustment means for adjusting an adjustableelement in a structure in which the drive unit is incorporated, thedrive unit being supplied with power from a power supply including atransformer having a primary side for connection to a mains voltage(alternating current) and a secondary side with rectification andsmoothing for connection to the DC motor, with a first control tocompensate for the loss in the motor and with a second control adaptedto remove variations in the voltage, thereby keeping the speed of themotor constant for a long period of time as well as for a short periodof time.

There is no evident explanation to the phenomenon, but a possibleexplanation is that the reason is the reduction in the variation of themoment supplied by the motor, thereby producing a more constantengagement of the individual parts of the transmission. This, in turn,leads to smaller vibrations in the motor and the transmission as well asthe structure as a whole, and thereby the acoustic noise is reduced.

It is possible to use various means for realizing the first control tocompensate for the loss in the motor. Here, mention may be madeindiscriminately of pseudo motors (used e.g. in WO 02/03526 to Linak A/Sfor synchronous running of two or more DC motors, where the principle inthe pseudo motor is shown in FIG. 3 and described on page 8, line 15 ff,hereby incorporated by reference in the present application), locking tocommutator noise, generators having a predetermined frequency andcontrol loop or Hall sensors. With the finding of the invention it iswithin the skills of a person skilled in the field to apply thesesolutions and similar solutions to achieve the intended effect.

A particularly favourable embodiment of the second control adapted toremove the ripple in the voltage is characterized in that the smoothingis performed by the following two steps, viz. a forward step and a powerstep, where the forward step may be described as the input voltageV_(in) from the rectification and a constant which is given by theactual embodiment of the circuits for the two steps. The power step maybe described by its output voltage V_(out) and the input voltage V_(in)and the duty cycle which is the proportional time for which the powersupply may be loaded during a given period of time. The result of thetwo steps gives V_(out)=k, i.e. the smoothed voltage is constant. Thisstructure is entirely or practically entirely free of ripple voltage andsimultaneously has a high efficiency and is moreover simple andinexpensive.

When long and short periods of time are mentioned, this should of coursebe seen in relation to the given adjustments, where an example of a longperiod of time is 30 msec. to 1 sec., while a short period of time isless than 10 msec., viz. the reciprocal of 100 Hz.

With the finding of the invention it is realized that the second controlmay basically consist of a capacitor having a great capacity exclusivelywith the purpose of removing variations in the voltage. This is a simplesolution having a high efficiency, but the price is high, and ripple isnot removed entirely. In addition, the physical size of the capacitor isa problem.

Another possibility is a linear control which is characterized by alow-cost, ripple-free and simple structure, but the power loss is quitegreat.

A further possibility is switch mode, which is favourable in that it isripple-free and has a high efficiency. However, the structure is complexand the price is high.

Examples of the preferred control for removing the ripple in the voltageas well as a couple of noise measurements will be described below withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a first example of a power step,

FIG. 1 b shows a second example of a power step,

FIG. 1 c shows a third example of a power step,

FIG. 2 shows an example of a forward step,

FIG. 3 shows a noise measurement performed on a lifting column with acommon power supply,

FIG. 4 shows a noise measurement on the same lifting column and powersupply, but designed in accordance with the invention, and

FIG. 5 shows a presentation of the embodiment when the second controlremoves ripples in the voltage so as to maintain the speed of the motorconstant for a shorter duration than the time period the first controlkeeps the motor speed constant by compensating for loss in the motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The first example is a Buck circuit, which may be expressed byV_(out)=V_(in)·duty cycle. The corresponding forward step must be dutycycle=k/V_(in). The result is then V_(out)=k.

The next example is a boost circuit, which may be expressed byV_(out)=V_(in)/duty cycle. The corresponding forward step must be dutycycle=V_(in)/k. The result is then again V_(out)=k.

The third example is a flyback circuit, which resembles the precedingone, but with the opposite sign V_(out)=−V_(in)/duty cycle. The sameforward step as before may be used, viz. duty cycle=V_(in)/k, whichgives V_(out)=−k.

FIG. 2 shows a forward step which may be used in connection with allthree examples of power steps, with just a dimensional adaptation to theexamples shown. It is seen to include an NE556 astable timer having aduty cycle directly controlled by output voltage with adjustments frominput voltage.

It will be appreciated that the invention is not limited to thementioned circuits, and that other circuits fulfilling the function, asstated in the claims, are just as suitable.

To illustrate the effect of the invention, reference is made to theresults of two noise measurements, where FIG. 3 shows a noisemeasurement made on a lifting column having a common power supply, i.e.transformer, rectification and smoothing with capacitor (in the presentcase a lifting column having the designation DL1 and a control boxhaving the designation CBD1 of the make Linak A/S). FIG. 4 shows a noisemeasurement of the lifting column and power supply, but designed inaccordance with the invention (in the present case a lifting columnhaving the designation DL1 and a control box CBD4 of the make LinakA/S). As will appear from the curves, there is a significant decline inthe noise, totally from 57.0 dB(A) to 50.6 dB(A).

FIG. 5 shows a presentation of when the second control maintains thespeed of the motor constant for a shorter time period than that of thefirst control.

Although the invention has been explained in the foregoing with respectto articles of furniture in the broadest sense, the invention is notrestricted to this, of course. In industrial and constructionapplications or applications in means of transportation there may alsobe a requirement for a low level of acoustic noise.

1. A drive unit comprising a DC motor having a rotor consisting of aplurality of coils connected to a commutator in connection with a set ofbrushes to establish a voltage across the coils, said DC motor, via atransmission, driving an adjustment means for adjusting an adjustableelement in a structure in which the drive unit is incorporated, a powersupply for driving said drive comprising a transformer having a primaryside for connection to a mains voltage (alternating current) and asecondary side with rectification and smoothing for connection to the DCmotor, a first control means to compensate for loss in the motor,thereby maintaining a speed thereof constant for a first period of time,a second control means that removes ripples in the voltage, therebymaintaining the speed of the motor constant for a second period of time,said second period of time being shorter in duration than said firstperiod of time, and including an astable timer having a duty cycle whichis controlled by output voltage and adjusted by input voltage, andwherein said first period of time is 30 msec. to 1 sec. and said secondperiod of time is less than 10 msec.
 2. A drive unit comprising a DCmotor having a rotor consisting of a plurality of coils connected to acommutator in connection with a set of brushes to establish a voltageacross the coils, said DC motor, via a transmission, driving anadjustment means for adjusting an adjustable element in a structure inwhich the drive unit is incorporated, a power supply for driving saiddrive comprising a transformer having a primary side for connection to amains voltage (alternating current) and a secondary side withrectification and smoothing for connection to the DC motor, a firstcontrol means to compensate for loss in the motor, thereby maintaining aspeed thereof constant for a first period of time, a second controlmeans that removes ripples in the voltage, thereby maintaining the speedof the motor constant for a second period of time, said second period oftime being shorter in duration than said first period of time, andincluding an astable timer having a duty cycle which is controlled byoutput voltage and adjusted by input voltage, wherein the second controlmeans provides a forward step in which a duty cycle is expressed by kand V_(in), and a power step in which V_(out) is expressed by V_(in) andthe duty cycle, wherein the result of the forward step and the powerstep is V_(out)=K, and wherein V_(in) is an input voltage from therectification, V_(out) is an output voltage from the power step, k is aconstant given by actual circuits for the forward step and the powerstep, and wherein the duty step is the proportional time for which thepower supply is loaded during a given period of time, wherein theforward step is given by duty cycle=K/V_(in), and the power step byV_(out)=V_(in)*duty cycle.
 3. A drive unit comprising a DC motor havinga rotor consisting of a plurality of coils connected to a commutator inconnection with a set of brushes to establish a voltage across thecoils, said DC motor, via a transmission, driving an adjustment meansfor adjusting an adjustable element in a structure in which the driveunit is incorporated, a power supply for driving said drive comprising atransformer having a primary side for connection to a mains voltage(alternating current) and a secondary side with rectification andsmoothing for connection to the DC motor, a first control means tocompensate for loss in the motor, thereby maintaining a speed thereofconstant for a first period of time, a second control means that removesripples in the voltage, thereby maintaining the speed of the motorconstant for a second period of time, said second period of time beingshorter in duration than said first period of time, and including anastable timer having a duty cycle which is controlled by output voltageand adjusted by input voltage, wherein the second control means providesa forward step in which a duty cycle is expressed by k and V_(in), and apower step in which V_(out) is expressed by V_(in) and the duty cycle,wherein the result of the forward step and the power step is V_(out)=K,and wherein V_(in) is an input voltage from the rectification, V_(out)is an output voltage from the power step, k is a constant given byactual circuits for the forward step and the power step, and wherein theduty step is the proportional time for which the power supply is loadedduring a given period of time, wherein the forward step is given by dutycycle=V_(in)/k, and the power step by V_(out)=V_(in)/duty cycle.